I believe the Hall effect happens inside the transmission. A rotor (with three arms, I think) on the output shaft spins near a pickup coil which generates a signal for the speedometer (in 560 SL/SEL/SECs, anyway).
All you see on the back of the speedo is four male electrical connectors (12v+, ground, signal FROM the trans, and signal TO the cruise control).
Another device that uses the same principle is the Pertronix ignition (which replaces mechanical points). A rotor with small magnets along the periphery replaces the stock rotor, and each time a magnet goes by the pickup, a small electrical current is created.
Think of the Hall effect in these applications as a small pulsed example of the phenomenon that makes a generator generate DC current.
M D "Doc" Nugent